环丁二烯是最简单的[n]-轮烯（[4]-轮烯），化学式为C₄H₄。它是一种极不稳定的碳氢化合物，仅能单独存在5秒钟。尽管环丁二烯含有交替的单双键，但是它不符合休克尔规则，因为它只有4个π电子，不符合4n+2的通式。由于金属原子可提供2个电子，所以有些环丁二烯-金属化合物实际上是稳定的。

由于环丁二烯的π电子总能量要高于相应脂肪烃——1,3-丁二烯，因此它被称为是反芳香性的。然而最近的红外光谱数据表明，环丁二烯并非正方形结构，而为长方形，而且1,2-二氘代-1,3-环丁二烯存在两个立体异构体。这说明其π电子具有部分定域性质，因而环丁二烯也不完全是反芳香性的。至于它不是正方形结构的原因，可以将它看作姜-泰勒效应在有机化学中的一个特殊应用。

Cyclobutadiene is the smallest [n]-annulene ([4]-annulene), an extremely unstable hydrocarbon having a lifetime shorter than five seconds in the free state. It has chemical formula C₄H₄. It is believed to be in equilibrium between a pair of rectangular/nonplanar ground states and a square, excited triplet state based upon theoretical calculations, and through spectroscopic and crystallographic investigation of substituted cyclobutadienes, in an argon matrix and inside ****. Though it has alternating single and double bonds, it is predicted triplet, unstable and antiaromatic by Hückel's rule, because its ring has 4 π-electrons, and 4 is not twice an odd number. Some cyclobutadiene–metal compounds are stable, thought to be caused by the metal atom providing 2 more electrons to the system. Planar, rectangular distortion to a singlet ground state is caused by the Jahn–Teller effect. Tetra-t-butyl substituted cyclobutadiene is sufficiently stable to gather x-ray crystallographic images of the structure which has a distorted, nonplanar configuration and double bond lengths longer than normal (1.464 vs expected 1.34)